1. Field of the Invention
The invention relates in general to an antenna structure with an antenna radome and a method for raising a gain thereof, and more particularly to an antenna structure, which has an antenna radome, a high gain and a simple structure, and a method for raising a gain thereof.
2. Description of the Related Art
Recently, the wireless communication technology is developed rapidly, so the wireless local area network (Wireless LAN) or the wireless personal area network (Wireless PAN) has been widely used in the office or home. However, the wired network, such as a DSL (Digital Subscriber Line), is still the mainstream for connecting various wireless networks. In order to wireless the networks in the cities and to build the backbone network appliance between the city and the country with a lower cost, a WiMAX (Worldwide Interoperability for Microwave Access) protocol of IEEE 802.16a having the transmission speed of 70 Mbps, which is about 45 times faster than that of the current T1 network having the speed of 1.544 Mbps, is further proposed. In addition, the cost of building the WiMAX network is also lower than that of building the T1 network.
Because the layout of the access points in the backbone network is usually built in a long distance and peer-to-peer manner. Thus, the high directional antenna plays an important role therein so as to enhance the EIRP (Effective Isotropically Radiated Power) and to achieve the object of implementing the long distance transmission with a lower power. Meanwhile, the converged radiating beams can prevent the neighboring zones from being interfered. The conventional high directional antenna may be divided into a disk antenna and an array antenna. The disk antenna has an extremely high directional gain, but an extremely large size. So, it is difficult to build the disk antenna, and the disk antenna tends to be influenced by the external climate.
When the required directional gain of the array antenna increases, the number of array elements grows with a multiplier, the antenna area greatly increases, and the material cost also increases greatly. Meanwhile, the feeding network, which is one of the important elements constituting the antenna array, becomes complicated severely. The feeding network is in charge of collecting the energy of each of the antenna array elements to the output terminal as well as to ensure no phase deviation between the output terminal and each of the antenna array elements. Thus, the problems of phase precision and transmitted energy consumption occur such that the antenna gain cannot increase with the increase of the number of array elements.
In 2002, G. Tayeb etc. discloses a “Compact directive antennas using metamaterials” in 12th International Symposium on Antennas, Nice, 12-14 Nov. 2002, in which the metamaterial antenna radome having a multi-layer metal grid is proposed. The electromagnetic bandgap technology is utilized to reduce the half power beamwidth (only about 10 degrees) of the microstrip antenna greatly in the operation frequency band of 14 GHz, and thus to have the extremely high directional gain. Based on the equation of c=f×λ, however, when the antenna is applied in a WiMAX system with the operation frequency band of 3.5 GHz to 5 GHz, the wavelength is greatly lengthened because the frequency is greatly lowered. Thus, the antenna radome has to possess the relatively large thickness correspondingly, and the overall size of the antenna increases. Meanwhile, the multi-layer metal grid acts on the far-field of the antenna radiating field, so the overall size of the antenna structure increases and the utility thereof is restricted.